1. Field of the Invention
The present invention relates to a vehicle heated window such as an automotive heated rear window, with a defogger for heating the window glass and thereby removing or preventing buildup of fog, ice or snow.
2. Description of the Related Art
In vehicles as automobiles, it is quite important to secure a good visibility for safety driving. Thus, defogging is indispensable not only to the windshield but to the rear window for paying attention to the rear at the time of driving in reverse gear or to the following cars. For this reason, a window glass having printed thereon a plurality of electric heating strips is used in the most automobiles.
It has heretofore been practiced to determine the distance between adjacent two of the heating strips at least at potions thereof disposed in the central region of the window glass so as to be included usually in the range of 29 to 33 mm.
This is because when the distance is too small the visibility in the rearward direction is deteriorated and when the distance is too large it is impossible to heat the window glass sufficiently. Heretofore, the resistance of the heating strips and the distance between adjacent two of them have been designed so that (I.sup.2 R)/DL is nearly constant where I is supply current per one heating strip, i.e., the quantity of generated heat per unit area in the central region of the window glass, R is resistance, D is the distance between adjacent two of the heating strips, and L is the length of the heating strips in the central region of the window glass.
However, in case a vehicle is equipped with a battery of a small capacity, it is inevitable to sacrifice the defogging ability. As a result, there may occur such a case in which the quantity of generated heat is designed to be smaller than that obtained by the above described expression. On the other hand, there may occur another case in which the quantity of generated heat is designed to be larger than that obtained by the above described expression in case a vehicle is equipped with a battery of a large capacity or a vehicle is of the kind having the heating strips for the purpose of melting ice mainly.
According to a number of experiments carried out by the inventors of this application, it was found that the heating strips designed according to the above described concept had a problem in that the quantity of supply heat for heating the window glass up to a predetermined temperature in a predetermined time was generally too large.